Apparatus for coupling power into a body of gas

ABSTRACT

Apparatus for coupling power into a body of gas by induction to provide a gas discharge, comprises a source of a/c power ( 3 ) coupled to a circuit arrangement having an inductor comprising two coils ( 1, 2 ), each constituted by a respective elongate conductor, the conductors each having one end coupled to the circuit arrangement at opposite respective sides of the inductor, and having the other end at a floating potential. The mutual capacitance between the coils, together with the inductance of the inductor as a whole, constitute a balanced self-resonant circuit having a resonant frequency which is greater than 1 MHz and less than 100 MHz. This arrangement can reduce temperature variations in the resonant frequency and can reduce unwanted r.f. emissions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus for coupling power into a body ofgas by induction to provide a gas discharge, comprising a source of a/cpower coupled to a circuit arrangement having an inductor comprising aplurality of turns of a conductor surrounding a space having a body ofgas. It relates particularly, though not exclusively, to coupling of a/cpower at radio frequencies.

2. Background

Radio frequency gas discharges are used in a variety of technical fieldssuch as lighting and thin film deposition and etching. Drive circuitsfor such gas discharges normally have a coupling inductor as part of aseries tuned circuit including a series capacitor. The circuit is drivenat its resonant frequency to produce the high voltage required to strikethe discharge. After striking the discharge, current in the inductorcouples into the conducting gas plasma by transformer action (i.e.inductively), the plasma thus loading the resonant circuit and reducingthe resonant voltage.

The series capacitor which is used in such drive circuits to completethe tuned circuit introduces several drawbacks. For example, to strikethe gas discharge or plasma, high voltages such as, for example, severalthousand volts may be encountered. Low cost capacitors have asignificant temperature dependence at such voltages, which can cause theresonant frequency to change. The placement of the capacitor can beproblematical because of the danger of breakdown of the printed circuitboard. In addition, the resonant node on the side of the capacitorconnected to the coil constitutes a “hot” end to the coil, increasingthe problem of r.f. radiation in operation.

A known apparatus for coupling power into a body of gas and minimisingthe problem of r.f. radiation is disclosed in U.S. Pat. No. 5,525,871

SUMMARY OF THE INVENTION

According to the present invention, there is provided apparatus asdefined in claims 1-4. This arrangement avoids the problems associatedwith a separate series capacitor by using the mutual capacitance betweentwo adjacent coils, and can minimise unwanted r.f. radiated energy.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying diagrammatic drawings, inwhich:

FIG. 1 shows a cross section of a first inductor according to theinvention,

FIG. 2 shows a cross section of a second inductor according to theinvention, and

FIG. 3 shows a cross-section of a third inductor according to theinvention

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

FIG. 1 shows a cross section through a first embodiment of theinvention. The inductor is made up of a first coil (1) and a second coil(2), the windings being interleaved to form a bifilar resonant circuit.The separate wires forming the two coils are shown as having a differentpattern of hatching in the Figure. The circuit also comprises a sourceof a/c power (3), and the Figure also shows a body of a gas (4) in whicha gas discharge is maintained by inductive coupling from the coils. Theinductive component of the resonant circuit is either the inductance ofthe first or the second coil, even although one end of each isunconnected to anything, and thus at a floating potential. Thecapacitive component of the resonant circuit is formed from thedistributed interwinding capacitances between the first coil and thesecond coil. This capacitance is in general not a strong function oftemperature.

In use, the r.f. current entering the first coil flows partly along thefirst coil and partly via interwinding capacitance to the second coil insuch a way that by the far end (floating end) of the first coil all thecurrent has transferred to the second coil, the current thus exiting viathe non-floating end of the second coil.

High resonant voltages are developed at the unconnected (floating) endsof the two coils, which should be insulated. The voltages at the twofloating ends of the coils are in anti-phase, thus reducing unwantedr.f. emissions if the ends are located close to one another, incomparison with the unbalanced conventional arrangement having anexternal capacitor.

A number of different practical winding arrangements can give aninterwinding capacitance which varies less with temperature than lowcost external capacitors.

Concomitant with the provision of a resonant circuit having a suitableresonant frequency, the coils/windings must provide an electric fieldsuitable for striking the plasma, and an inductive field suitable forefficient inductive coupling to the gas plasma. FIGS. 2 and 3 show twofurther embodiments of windings in accordance with the present inventionwhich give significantly different electric field profiles compared withthe arrangement shown in FIG. 1. These arrangements may be advantageousfor starting (striking) the discharge.

Examples of the arrangement shown in FIG. 1 have been constructed bywinding 12 turns of PTFE coated wire 0.25 mm in thickness for each ofthe two coils directly onto a transparent glass envelope 28 mm indiameter containing neon for use as an electrodeless lamp. In thisexample the resonant frequency was 10.9 MHz, and the inter windingcapacitance was of the order of 50 pF. It is anticipated that inductorshaving a suitable inter coil capacitance will be capable of beingconstructed for operation at resonant frequencies between approximately1 and 100 MHz.

The coils preferably comprise a bifilar winding, the two coils acting asa single RF coupler in use, with one end of each coil being left opencircuit. An advantage of such an arrangement is that electrostaticdamage caused by the build up of electrical fields on insulative partsof the apparatus in use is minimised.

What is claimed is:
 1. Apparatus for coupling power into a body of gasby induction to provide a gas discharge, comprising: an input for a/cpower coupled to an inductor comprising a plurality of turns of aconductor surrounding a space having a body of gas, the inductorcomprising two coils, each of the two coils comprising a respectiveelongate conductor; wherein the two coils each has one end coupled tothe input for a/c power at either side of the inductor, and each has theother end at a floating potential.
 2. Apparatus as claimed in claim 1,wherein: a mutual capacitance between the two coils, and an inductanceof the inductor as a whole, comprise a balanced self-resonant circuit.3. Apparatus as claimed in claim 1 or claim 2, wherein: a frequency ofa/c power input to the input for a/c power is greater than 1 MHz andless than 100 MHz.
 4. Apparatus as claimed in claim 1, wherein: the twocoils are arranged to comprise a bifilar winding.
 5. Apparatus asclaimed in any preceding claim, wherein: the apparatus is included in adrive circuit of an electrodeless lamp.
 6. Apparatus as claimed in claim1, further comprising: an a/c power source coupled to said input for a/cpower.